The present invention relates to a power tool and a gear unit assembled therein.
In a power tool, vibration force due to alternating current, pitch error and eccentricity of gears or the like cause rotational fluctuation. Due to the rotational fluctuation, the tooth surfaces of gears on the drive shaft side and driven shaft side repeatedly bump against each other, thereby causing noise. Further, the collision force between the tooth surfaces produces vibration force to generate noise at the portions other than gear parts. Particularly loud noise is generated due to large impact force, since large torque is provided at the drive shaft side at an initial start-up period.
To avoid the above-described problem, improvement on dimension accuracy of the gear or on a design in shape of the tooth surface have been proposed. Further, in order to structurally reduce the noise, a gap is positively provided between a steel key and an inner peripheral side of a drive gear, the steel key being interposed between the inner peripheral side of the drive gear and an outer peripheral side of a drive shaft for coupling together. However, the steel key may be deformed due to long-term use thereby generating larger gap, which may adversely lead to noise.
Laid open Japanese Patent Application publication No. H03-49883 discloses an elastic member adapted for power transmission and interposed between a drive gear and a drive shaft. However, the elastic member may be squashed and become ineffective when excessive load is applied between the drive shaft and drive gear.